Corned beef is made from brisket that is cured over multiple days in a salt and sugar mixture seasoned with pickling spices like peppercorns, cloves, coriander etc.
To brine the brisket bring a large pot of water over medium heat and stir in both salts, sugar, garlic, bay leaves, cloves, mustard seeds, coriander seeds, peppercorns, and allspice, stirring until the mixture is warm and the salt and sugar is completely dissolved.
Let the brine come to room temperature then add to a container large enough to fit the brine and the brisket. Add the brisket and weight the meat down with something heavy like a plate. Cover and refrigerate for 5 days, turning once a day.
When ready to cook the brisket, remove it from the brine and rinse it completely.
Heat oven to 325ºF (16s0ºC). Place brisket in a baking dish large enough to hold the brisket. Arrange onion around the side and pour the beer over the top. Add just enough water until the brisket is covered.
Tightly cover with aluminum foil and transfer to the oven. Let cook until the tender enough that a fork can easily be inserted into the meat, about 3 1/2 to 4 hours.
Let cool then remove to a cutting board and thinly slice. Keep the meat in the cooking liquid until ready to use, for up to 3 days.
For the Russian dressing:
Add all ingredients to a small mixing bowl and stir until combined. Cover and refrigerate until ready to use. Can be made up to a week in advance.
For assembly:
Heat oven to broil. Add the slices of bread to a baking sheet. Broil until the slices of bread are toasted on both sides, flipping halfway through, about 1 minute. Remove the bottom slices to the side and add 1 to 2 slices of Swiss cheese to the other two slices of bread. Broil until melted, about an additional minute.
Spread 2 tablespoons (30g) of the Russian dressing over each of the bottom slices of bread followed by slices of corned beef and ¼ cup (40g) of sauerkraut. Close the sandwiches and slice in half. Serve immediately with a side of pickles.